1. Field of the Invention
This invention relates to signature recognition systems for providing articles with distinctive signatures and means for verifying those signatures.
2. Discussion of Prior Art
Patent Application no. GB 2235287 B discloses an optical sensor based on the use of surface plasmon polaritons (SPP). The sensor comprises apparatus for detecting a surface plasmon-polariton resonance maximum which occurs following polarisation conversion of particular wavelengths of radiation incident upon a surface which correspond to the excitation of an SPP at or about its resonant frequency.
Bar code systems are well known as a means of distinguishing certain items and are easily read using light pens. As a two dimensional system, bar codes are easily distorted by smudges of dirt, creases, scratches and so on, this can cause errors in readings taken by a light pen. Furthermore, as they are visible to the naked eye, conventional bar code systems are fairly simple to copy or alter.
Magnetic strips and reading devices are also commonly used as a security measure for identifying personal identification cards, credit cards and the like. Like conventional optical bar codes, these strips are easily damaged by bending or scratching and can also be affected by close contact with other magnetic sources.
The present invention is a signature recognition system for identifying an article with a distinctive diffractive element (or elements) and verifying the presence of that element or elements comprising;
an article with one or more diffraction gratings impressed thereon, the grating(s) exhibiting periodic wave surface profile having a depth-to-pitch ratio xcex4 of between 0.1 and 0.5,
a source of polarised electromagnetic radiation of wavelength xcex such that the pitch G of the periodic wave surface profile of the grating(s) is comparable to an integer multiple n of that wavelength
means for directing the source of polarised electromagnetic radiation to the surface of the grating(s) at a plane of incidence substantially normal to the plane of the surface of the diffraction grating and at an angle of approximately 45xc2x0 azimuth to the alignment of the grooves on the surface of the diffraction grating, and
means for detecting radiation reflected from the grating(s) surface which is oppositely polarised to the incident radiation.